def prost(x):
    k = 0
    for d in range(1,x+1):
        if x%d==0:
            k+=1
    if k == 2:
        return True
    else:
        return False
maxx = 0
num = int(input())
if num >= 1000 and num <= 9999:
    n1=str(num)
    for cifra in range (len(n1)):
        if prost (int(n1[cifra])) == True:
            maxx = max(maxx,int(n1[cifra]))
    for chislo in range (len(n1)-1):
        if prost(int(n1[chislo]+n1[chislo+1])) == True:
            maxx = max(maxx,int(n1[chislo]+n1[chislo+1]))
    for chislo in range (len(n1)-2):
        if prost (int(n1[chislo]+n1[chislo+1]+n1[chislo+2])) == True:
            maxx = max(maxx,int(n1[chislo]+n1[chislo+1]+n1[chislo+2]))
    for chislo in range (len(n1)-3):
        if prost (int(n1[chislo]+n1[chislo+1]+n1[chislo+2]+n1[chislo+3])) == True:
            maxx = max(maxx,int(n1[chislo]+n1[chislo+1]+n1[chislo+2]+n1[chislo+3]))

    print(maxx)
else:
    print(0)
